The present invention relates to a switching control method which can smoothly effect transfer from a pressure holding step to a metering and kneading step in an injection-molding machine employing a servomotor as an injection shaft driving source.
An injection-molding machine comprises an injection shaft driving source for axially driving a screw, and operates in such a manner that the screw is driven for forward movement to inject molten resin into a mold. After the molten resin is injected into the mold, a hold pressure, which lower than the injection pressure and which acts in the same direction is as the injection pressure applied to the screw. Typically in an injection-molding machine which employs a servomotor as an injection shaft driving source and which comprises a numerical control unit including a servo circuit for driving the servomotor, a move command sufficient that the tip of the screw reaches a tip of a heating cylinder is sent to the servo circuit to generate the hold pressure. That is, the screw is driven for forward movement by axial driving force in accordance with the move command and, subsequently, the screw is retained at an axial position short of the cylinder tip position by a cushion amount, where the axial driving force balances with the pressure of the molten resin in the mold. This is the hold pressure that is applied to the resin. During a subsequent metering and kneading step, axial pressure, i.e., back pressure lower than the hold pressure and acting in the backward direction is applied to the screw while rotating the latter, to effect metering and kneading.
At transfer from the pressure holding step to the metering and kneading step, the molten resin slightly expands in volume due to a differential pressure between the hold pressure. The back pressure, and the screw moves rearwardly in response to this differential pressure. At this time, an error amount equal to an added value of the cushion amount and the amount of rearward movement of the screw is accumulated in an error register of the servo circuit. As the error amount exceeds the register capacity, the error register overflows and, in response thereto, the typical numerical control unit generates an alarm and operates so as to interrupt operation of the injection-molding machine. It is inconvenient that the injection-molding machine stops in operation during execution of the usual injection molding cycle. Additionally, as is publicly known, even if transfer is made to the metering and kneading step after follow-up of the error register at completion of the pressure holding step so as to bring the error amount accumulated in the error register and equivalent to the cushion amount, to "0", similar inconvenience is caused if the amount of rearward movement of the screw is large.